I was under a similar impression, so I don't blame you for being confused. As long as the language is something that could reasonably exist, doesn't grant any advantages to the player, isn't something magic-based or what have you, then it would be fine. But then, I was told by the co-founder that since the area where I'm staking my claim is surrounded by Indo-European languages, my language should be Indo-European-based. Then, I was later told by the founder I should request a ruling on the matter, hence why this topic is here.
I suppose this could loosely fit into a Corpis Juris sort of rule, but it seems unfair for a number of reasons. One, there are no factbooks or other sources of information for the surrounding regions on the western continent of Arcadia; I would've never known the languages were Indo-European based if I hadn't asked or requested more information and by that point I was already developing history with another member of the forums.
Two, I've already pointed out why a language with a logographic writing system could still reasonably exist in the area, especially when one considers how large the landmass of the western continent is, and the fact that the divides where different ethnicities and languages have not apparently been established and could theoretically be anywhere since this is an alternate kind of Earth that is very, very geographically different from the real Earth.
Three, it seems a bit misleading to be told Artemia is a community-built region if new members of the community have arbitrary, unwritten rules enforced upon them when they do try to build up the world.